Well what type of hens? A buff drake will not cause any harm to 5 duck hens. I have mainly noticed problems with drakes and chickens, they seem to do alright around larger poultry like turkeys. When the mating season comes the drake's body is pumped full of hormones and they become disgusting sex machines. I have seen them grab chicken hens by the neck, pull on their combs, and rip off their feathers if a rooster mounts the hen near the drake. I have also had drakes corner chicken hens and rape them, they get covered in mud, they lose tons of feathers, and are covered in scratches from the drake's nails. Because of this I can't even let the my drakes free range with the chickens during the breeding season.
Again, your drake will be fine if kept with duck hens, the hens will be fine too. The drakes usually do very well with chickens after the breeding season too. Around here the breeding season usually starts around March, that is when the hens start pumping out eggs and the drakes start to become violent with other poultry. It usually lasts till June or July.
Do you have chickens in the same area you are planning to keep the ducks?